The present invention relates to a brake booster provided with a booster piston mechanically associated with a master cylinder piston and a valve device disposed internally of the booster piston connected with a brake pedal, the valve device providing a controlled supply of pressure medium into a boosting chamber disposed between the booster piston and the brake pedal.
Such a brake booster is disclosed in British Pat. No. 1,322,156. The brake booster described in this patent has two reaction devices by means of which reaction forces act on the brake pedal. Upon the onset of the braking operation, only a hydraulic reaction device will be active which generates a reaction force via a piston and a spherical valve member controlling the supply of pressure. If the booster piston has moved by a certain amount it will abut against a rubber reaction disc by means of an outer rim, thereby, causing the rubber disc to bulge towards the brake pedal and, hence, displacing a reaction plunger which exerts a reaction force on the brake pedal via pins passed through the booster piston.
The hydraulic reaction device in the above brake booster is dimensioned such as to generate only a very small reaction force. In consequence, when the brake is operated, a certain pressure is built up in the brake system almost immediately as soon as the spherical valve member of the brake booster has opened. This so-called springing effect is desired in order to ensure that the brake shoes of the drum brakes, kept in rest position by return springs, will be brought into abutment against the brake drums at the beginning of the braking action without the driver, due to an increasing reaction force at the brake pedal, thinking wrongly that a braking force is already being exerted.
The design of the brake booster according to the above-cited British Patent is too complicated and its manufacture is, therefore, expensive. As far as its mode of operation is concerned, it is disadvantageous that a considerable brake pedal travel is needed before the brake shoes engage the brake drums. This results in a reduction of the maximum brake pedal travel available for braking.
This is no disadvantage in the event of auxiliary power operation since, due to the high pressure of the auxiliary energy, the transmission ratio can be selected so that even short displacement distances of the brake booster piston and, hence, of the brake pedal will permit the building-up of a pressure which will be high enough. However, in case the auxiliary energy fails the brake pressure must be entirely generated by the force of the driver's foot. A small transmission ratio, however, prevents the brake pdeal force from producing a sufficiently high braking pressure, said pedal force being considerably weaker than the auxiliary energy.